1. Field of the Invention
The present invention relates to annealing techniques, and in particular to an improved laser annealing device.
2. The Related Arts
Annealing is a commonly used heat treatment process of metals, which gradually heats the metal to a predetermined temperature, which is then maintained for a sufficient period of time and then cooled with an appropriate speed. The purposes of annealing is to reduce hardness, improve machinability, release residual stress, stabilize size, suppress the tendency of deformation and cracking, refine grains, adjust organization, and remove organization defects.
Annealing is also used in the semiconductor technology. For example, a semiconductor chip must be subjected to annealing after injection of ions. This because in injecting impurity ions into a semiconductor, the injected ions that are of high energy impinge the atoms on the crystal lattice of the semiconductor, causing displacement of some lattice atoms. Consequently, a great number of voids are formed, disordering atom arrangements in the injection area or converting it into a non-crystal zone. Thus, after the injection of ions, the semiconductor must be annealed at a predetermined temperature to restore the crystal structure and remove the defects. Further, annealing also provides the functions of activating donor and acceptor impurities, namely causing some impurity atoms that are located at gaps into substitution positions through annealing. The temperature of annealing is generally 200-800° C., which is much lower than the temperature for thermal diffusion doping. Annealing is also carried out after evaporation of electrode metals in order to have the surface of the semiconductor bonded with the metal to form an alloy for forming good contact (reducing contact resistance). Under this condition, the annealing temperature is selected to be slightly higher than metal-semiconductor eutectic point.
Laser annealing is a new technique of semiconductor processing and provides a much better result than the regular thermal annealing. After laser annealing, the impurity substitution rate may get as high as 98-99%, making the resistivity of poly-silicon lowered to around ½-⅓ of the regular thermal annealing and also highly improving integration of an integrated circuit to reduce the spacing between circuit components to 0.5 microns. Laser annealing is generally carried out with two methods. The first method uses a continuous laser, such as argon ion laser, which applies a focused light beam to the semiconductor material. The crystallization process of the semiconductor material is that it is melted first and then the semiconductor gets gradually solidified due to the slope energy distribution and movement of the beam. The second method uses pulse oscillation laser, such as excimer laser. The crystallization process of the semiconductor is that the semiconductor is instantaneously melted by the high energy laser pulse and gets solidified.
The laser head for laser annealing is relatively expensive and replacing is time-consuming and may thus affect the manufacturing operation. Heretofore, most of the laser heads for annealing purposes are provided with certain protection means. U.S. Pat. No. 6,087,277 provides a laser annealing device with protection means. Referring to FIG. 1, a schematic view showing a conventional laser annealing device with protection means is given. The laser annealing device comprises top and bottom casings (not shown). The top forms a transparent output window 10 for laser beam. A laser beam 30 travels vertically through the laser beam output window 10 to carry out annealing on a metal layer formed on a substrate 20 located at the bottom. Usually, the laser beam 30 is caused to make a scanning movement on the substrate 20. The irradiation of the high energy laser beam 30 on the material of the substrate 20 to be annealed may easily cause outward spreading of scraps 40. The direction of spreading of the scraps 40 are indicated by arrows of FIG. 1. After long operation of the annealing device, the scraps 40 gets accumulated on the undersurface of the laser beam output window 10, making it not transparent and thus affecting the transmission of the laser beam. It is thus required that the laser beam output window 10 be frequently replaced and this increases the cost. To protect the laser head, the laser annealing device disclosed in U.S. Pat. No. 6,087,277 provides a mask 50 that is located between the laser beam output window 10 and the substrate 20 and is movable in synchronization with the laser beam 30 to prevent the scraps 40 from affecting and contaminating the laser head. The mask 50 is set horizontally and forms in a center thereof a slit 60. The laser beam 30 that travels down vertically passes through the laser beam output window 10 and then the slit 60 before it irradiates the substrate 20. Thus, the mask 50 prevents the scraps 40 to get accumulated on the laser beam output window 10 and allows the period of replacement of the laser beam output window 10 to be extended. Yet, further improvement can be made on the known laser annealing device to reduce the accumulation of scraps and lower down the number of replacing the laser head.